A conventional electronic device includes a power conversion circuit to convert a frequency or a voltage of an arbitrary input power source into frequencies or voltages for various devices in the electronic device. Particularly, a switching method is widely used to make the power conversion circuit compact and efficient. However, in the power source circuit adopting the switching method, a switching noise having a high frequency is generated by the switching operation, thereby causing a terminal noise or an electromagnetic wave noise.
As a noise filter for reducing such an electromagnetic noise, Japanese Patent Application Laid-Open No. 2008-182784 discusses a line bypass capacitor (hereinafter, referred to as a “Y capacitor”). FIG. 9 is an example of a filter circuit diagram used for a conventional switching power source circuit.
A filter circuit 190 illustrated in FIG. 9 includes Y capacitors 121 and 122 connected between each supply line of a pair of supply lines and a ground (hereinafter, referred to as a “GND)”. The Y capacitors 121 and 122 function to discard to the GND a common mode component of the high frequency switching noise generated from a switching element to restrain the terminal noise or the electromagnetic noise.
Impedance of the two supply lines connected from an input power source to the GND is generally set to be equal so that the two supply lines are balanced with respect to the GND. For example, in a filter circuit 190 illustrated in FIG. 9, the two Y capacitors 121 and 122 keep balanced by having capacity values thereof as a same value.